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Tracing Human Evolution to its Roots

Molecular evidence suggests that the human line split from the chimp line only about 6 million years ago. Some recent fossil finds date from between 4.1 and 6 million years ago, placing them very close to that evolutionary divergence. Evidence from fossil animals, plants, and soils associated with these early hominids indicate that the environment in which they died was quite densely forested, prompting a reassessment of some of the hypotheses that have been suggested for the origins of human bipedalism.

Credits: From Biology, by Kenneth R. Miller and Joseph Levine, Copyright 2002 by Pearson Education, Inc. Reproduced by permission of the publisher.

Tracing Human Evolution to its Roots

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Tracing Human Evolution to its Roots:

A series of new fossil discoveries in the 1990s generated great interest among scientists involved in research into human origins, pushing the date for the beginnings of bipedalism back to more than 4 million years before the present -- and into a rather different context from the savannah environment which earlier research had suggested.

In 1994, an international team led by paleontologist Tim White announced that they had found a new species of hominid in Ethiopia, dating from 4.4 million years ago. So distinctive were these fossils that the researchers decided that they came not only from a new species, but from a new genus as well, and gave them the name Ardipithecus ramidus. ("Ardi" means ground or floor in the local Afar language, and "ramid" means "root.")

The teeth of this species are more apelike than those of Australopithecus afarensis fossils like Lucy, and its brain was small. The intriguing question, though, is how it got around. Did Ardithecus ramidus walk bipedally, like modern humans? The fossil was encased in a hard matrix of rock, and the painstaking process of preparing and analyzing it is not yet complete. But some features of the skull suggest that it had some form of upright posture.

If A. ramidus was indeed a biped, a few of the hypotheses that have been suggested for the origin of bipedalism may have to be reassessed. Most of these ideas were based on earlier research which suggested that upright walking evolved at a time when open grasslands were replacing dense forests in the region. But A. ramidus was found with fossil seeds, plants, and animals which show that the environment was densely wooded.

Less than a year later, a team led by Maeve Leakey and her colleagues uncovered a new species of hominid, which they named Australopithecus anamensis (from "anam," the word for "lake" in the local Turkana language). The fossils were found at two sites in Kenya, Kanapoi and Allia Bay, in the Lake Turkana region. They were found in the sediments of an ancient lake, along with animal fossils that suggest that the environment was a river and gallery forest, grading into more open woodland. Other fossils from Kanapoi include fish and
reptiles. At 4.1 million years old, this new find predates Lucy's species, A. afarensis, by more than half a million years, and features of the leg bones definitely indicate that A. anamensis stood upright and walked bipedally.

In 2000, a team led by Brigitte Senut and Martin Pickford reported a 6-million-year-old fossil that they named Orrorin tungensis. While its discoverers are confident that Orrorin is a hominid, not every expert is convinced by the evidence presented so far. Some feel that it may prove to be a chimp ancestor -- which in itself would be exciting, since so few fossil apes are known -- or even the common ancestor of chimps and humans.

More recently still, Yohannes Haile Selassie, a colleague of Tim White, announced the discovery of another fossil which he named Ardipithecus ramidus kadabba, a distinct sub-species of A. ramidus. "Kadabba" means "oldest ancestor" in Afar, and indeed, at between 5.2 and 5.8 million years, A. r. kadabba, like Orrorin, is very close to the date of the split between chimps and humans established by molecular evidence. The fossil animals, plants, and soils at the site show that A. r. kadabba lived in a forested landscape, and details of a toe bone show that it was bipedal, although its gait was probably quite different from anything seen today.

Until more fossils of A. r. kadabba and Orrorin are found and further analysis is completed, it will not be clear how they moved around or just where they fit into the overall picture of human evolution. But there is no doubt that with these latest finds we are getting very close to the root of the human family tree.

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